1. Field of the Invention
This invention concerns oxygen enriched air/fuel combustion steam generation apparatus and methods, more especially in terms of reduced heat transfer surface areas as the percentage of oxygen in the combustion air is increased to 100 percent. In this way a new design of steam generation apparatus is promoted, characterized both by lower investment costs and operating costs.
2. Related Art
The steam generation research and development community faces an important challenge in the years to come: to produce increased amounts of energy under the more and more stringent constraints of increased efficiency and reduced pollution. In order to fulfil both of these requirements, oxygen-enriched air/fuel combustion appears like an attractive candidate, since it has already proven to lead to significant process improvements in other industrial applications, such as fuel savings, production increase or reduced emissions.
U.S. Pat. Nos. 6,282,901 and 6,314,896 disclose methods of oxygen enrichment in existing air/fuel combustion steam generation apparatus, involving a certain ratio between the oxygen enrichment and the flue gas recirculation, such that the heat transfer patterns are maintained relatively unchanged. The purpose of the present invention is quite different, since the present invention aims at creating a new design of steam generation apparatus, specially adapted for oxygen-enriched air/fuel combustion, preferably with oxygen enrichment higher than 90%. As used herein the term xe2x80x9cboilerxe2x80x9d will be used to denote generic steam generation apparatus, which includes boilers producing steam for power generation through turbines, as well as steam generation for other uses. U.S. Pat. No. 5,265,424 discloses and advanced furnace boiler system using oxygen as the oxidant. However, the patent fails to teach how to calculate heat transfer areas for the various heat transfer surfaces, and thus does not even mention or recognize the reduction in heat transfer surface areas possible using oxygen-enriched air, or industrially pure oxygen.
Oxygen-enriched combustion (OEC) has become a popular technique employed in a series of industrial applications, such as glass, steel, aluminum and cement manufacturing, to name only a few. The employment of the oxygen-enriched technique has proven to lead to significant process improvements in these industries, such as fuel savings, production increase, waste processing, and the like. Presently, there are applications where the employment of oxygen enriched-combustion has not yet started to be applied on a large scale. One of these applications is boilers, where very large amounts of fuel are used for combustion purposes.
Existing steam generation apparatus have widely ranging steam outputs, requiring an energy input from a few hundred kW to thousands of MW. However, the very large investment required for a new boiler, together with the already high thermodynamic efficiency of existing boilers make the introduction of operational changes relatively difficult to implement. The boiler operators are reluctant to introduce modifications in the boiler characteristics, due to possible changes in water vapor properties (temperature, humidity, and the like). Different heat transfer patterns in the various areas of the boiler (combustion space, convective regions) will lead to different local vaporization/superheating rates of the steam, with direct impact on the boiler tubes. Local vapor superheating may lead to lower heat transfer coefficients, therefore to local boiler tube overheating, eventually causing cracks in boiler tubes. It is therefore crucial, when retrofitting an existing air/fuel combustion boiler to combust oxygen-enriched air with fuel, to maintain relatively unchanged the heat transfer patterns as originally designed, in order to produce safely the designed vapor throughput.
Basically, the use of oxygen-enriched combustion has two consequences to the oiler: it reduces the mass fraction of nitrogen, and it increases the adiabatic temperature of the flame. It is thus clear that the oxygen-enriched combustion can dramatically affect the heat transfer patterns in a system characterized by both radiative and convective heat transfers. While the increased flame temperature has a beneficial role on the radiative heat transfer, the diminished flow rates and temperature levels in the convective part of the installation may lead to lower heat transfer rates in this region. This means that for systems where the radiative heat transfer is the main heat transfer mechanism, such as cement kilns or glass furnaces, the oxygen enrichment can be used as such, without further modifications. However, for systems where convective heat transfer is important, changes to the installations have to be performed, in order to maintain the design parameters of the system unchanged, without modifying the heat exchanger structure.
Several inventions have already dealt with oxygen enrichment in steam generating boiler operation, promoting different methods to retrofit existing installations. The oxygen boosting can actually be used in connection with:
Increase of production (steam throughput), with the same boiler design;
Redesign of the convective part, to ensure the same production;
Fuel staging, allowing gas temperature and mass flow rate increase, in order to maintain the same convective heat transfer as in the initial design.
Flue Gas Recirculation, to maintain general heat transfer patterns essentially the same as the air-based combustion.
None of these solutions have been readily accepted in the art, for one or more reasons. Therefore, there exists a need in the art for a new boiler design to apply oxygen-enriched combustion in steam generators. There is especially needed designs which allow taking advantage of the usual benefits of oxygen-enriched air/fuel combustion, while avoiding the above risks and constraints linked to the retrofit of existing boilers.
In accordance with the present invention, steam generation apparatus and methods are presented which overcome many, if not all of the above shortcomings of previously known systems.
A first aspect of the invention is a steam generation apparatus dedicated to combustion of a fuel with an oxygen-enriched oxidant, wherein the oxygen concentration of the oxidant may range from just above 21 percent to 100 percent, the apparatus comprising:
a) an oxidant preheater for exchanging heat with a flue gas, the preheater having a geometry, a size and a heat transfer area to take advantage of i) a flue gas flow rate that is lower, and ii) a flue gas temperature that is higher, due to oxidant/fuel combustion, than a comparable power air/fuel combustion boiler base case;
b) means for introducing a fuel and the oxidant into a combustion space within the furnace of the boiler and combusting the fuel in the presence of the oxidant in order to generate the flue gas;
c) the furnace having a radiant heat transfer section with a plurality of radiant heat transfer section tubes, said tubes having reduced heat transfer area compared to the base case (preferably resulting in less heat loss and higher efficiency than the base case); and
d) a convection heat transfer section having a plurality of convection heat transfer section tubes positioned so as to afford increased heat transfer between the flue gas and boiler feed water traversing therethrough while using less heat transfer area over the base case of air/fuel combustion.
More precisely, the steam generation systems of the invention are based on oxidants selected from the group consisting of oxygen-enriched air and oxygen, and are preferably characterized by reduced heat transfer areas, and preferably include in their basic configuration the same major components as in air/fuel combustion steam boilers, namely:
one or more oxidant preheaters through which oxidant flows and exchanges heat indirectly with flue gas, the flue gas having a first flue gas temperature, and creating a preheated oxidant stream which flows through a first conduit to the furnace of the boiler and a flue gas stream having a second flue gas temperature which is lower than the first flue gas temperature, the one or more preheaters adapted to be compatible with heated oxidant having an oxygen concentration ranging from just above 21 percent to about 100 percent;
a second conduit connecting a fuel source and the furnace of the boiler;
a boiler having a radiant section adapted to accept oxidant having an oxygen concentration ranging from just above 21 percent to about 100 percent, the radiant section having a plurality of radiant section tubes, and a convection section adapted to accept flue gases from combustion of a fuel and the oxidant, the convection section having a plurality of convection section tubes, each one of the plurality of radiant section tubes connected to one of the plurality of convection section tubes, the flue gases flowing eventually to the one or more oxidant preheaters through a third conduit;
a source of boiler feed water connected to the plurality of convection section tubes by a fourth conduit, the fourth conduit including an economizer that allows heat transfer between the flue gas at a third flue gas temperature, the third flue gas temperature higher than the first flue gas temperature, and the boiler feed water; and
a fifth conduit connected outside of the boiler enclosure to the plurality of convection or radiant section tubes and routing the steam flow to means for reducing pressure selected from the group consisting of control valves and turbines.
Preferred apparatus of the invention are those including at least one superheater, the superheater connected to at least a portion of the plurality of convection section tubes, thus allowing heat exchange between flue gas at a fourth flue gas temperature, the fourth flue gas temperature being greater than the third flue gas temperature, and steam flowing through the superheater; apparatus including a steam drum, the steam drum connected to at least a portion of the plurality of radiant and convection section tubes; and apparatus including an attemperator, the attemperator connected to the superheater.
Other preferred apparatus in accordance with this aspect of the invention are those wherein the oxidant preheater is selected from the group consisting of tubular, flat plate (recuperative), and regenerative heat exchangers, among others.
Yet other preferred apparatus are those wherein the boiler is a supercritical pressure boiler, apparatus wherein the boiler is a subcritical pressure boiler, and apparatus wherein water circulation in a subcritical pressure boiler is produced by a mechanism selected from the group consisting of gravity circulation, forced circulation, and combinations thereof.
Other preferred apparatus include a first expansion turbine for producing electricity from a first high pressure fluid flowing from the superheater, the first expansion turbine producing a low pressure fluid which is routed to a reheater, the reheater allowing heat exchange between flue gas at a fifth flue gas temperature, the fifth flue gas temperature greater than the fourth flue gas temperature, and the low pressure fluid flowing through the reheater, thus creating a second low pressure, reheated fluid which is subsequently routed to a second expansion turbine. Multiple turbines and associated reheaters are envisioned to be within the invention.
A second aspect of the invention is a method of operating the steam generation apparatus of the first aspect of the invention having reduced heat transfer area and increased efficiency for a power production equivalent to an air/fuel combustion base case, the methods comprising the steps of:
a) flowing the oxidant through the oxidant preheater and exchanging heat indirectly with the flue gas exiting the boiler, thus forming a preheated oxidant, the oxidant selected from the group consisting of oxygen-enriched air and oxygen;
b) introducing the fuel and the preheated oxidant into the furnace of the boiler and combusting the fuel with the preheated oxidant to generate the flue gas and thermal energy, the flue gas having a flow rate that is reduced compared to the base case; and
c) feeding the boiler with boiler feed water and circulating the water through the plurality of convection heat transfer section tubes and the plurality of radiant heat transfer section tubes, in order to preheat and evaporate the water, and produce superheated steam by heat transfer between the flue gas and the boiler feed water.
Further aspects and advantages of the invention will become apparent by reviewing the description of preferred embodiments that follow.